Preheat the oven at 170ºC/340ºF conventional oven and prepare a 2 x20 cm (8-inch) baking pan with parchment paper.

Sift together the all-purpose flour, cornstarch, baking powder, baking soda and salt and set it aside.

In a large bowl using an electric hand mixer or in a bowl of a stand mixer, add the butter, and granulated sugar and mix it on high speed for 3 minutes.

Add the eggs one at a time into the mixture and mix it in until combined.

Continue by adding half the dry ingredients, on low speed, mixing until just combined.

Then add the sour cream, vegetable oil, vanilla extract and mix until just combined. Then add the other half of the dry ingredients and mix until combined.

Using a rubber spatula, give the batter a gentle fold to make sure all of the ingredients have combined.

Pour the cake batter into the prepared pans and bake for 30-33 minutes or until a cake tester comes out clean.

Let the cakes cool on a cooling rack. After 10 minutes, gently remove the cake from the cake tin and let the cakes cool down completely.

Sift the powdered sugar and set it aside.

Mix the butter in a stand mixer with the paddle attachment or with a hand mixer for 4 minutes on medium/high speed. Scrape down the sides of the bowl and mix for an additional 2 minutes.

Add the powdered sugar, vanilla extra and milk in 2 parts. Whipping until each addition is fully incorporated. Optionally add vanilla paste to enhance the vanilla flavor even more.

Scrape down the sides of the bowl and give the buttercream a final mix for 2 minutes.

Start by cutting the very top of the cooled down cake layers so they have an even top. Place the first cake layer on your serving dish or cake board on your turntable. If you don’t have a turntable, don’t worry, you can easily decorate the cake without it.

Add 2 big scoops of buttercream and with an offset spatula, even it out. Then add the other cake layer and cover the whole cake in a thin crumb coat. Add the remaining buttercream, and even it out over the cake. Optional, make a wavy design with the buttercream, using the offset spatula and decorate with the sprinkles.